In a known electrically assisted bicycle including a motor, power is fed from a power storage such as a battery to the motor, a human driving force, which includes a pedal force applied to a pedal, is detected by a torque sensor, and an auxiliary driving force (assisting force) of the motor is added according to the human driving force. Thus, such an electrically assisted bicycle can smoothly travel on an uphill or the like.
In such an electrically assisted bicycle, a motor drive unit including a motor is disposed near a crankshaft. Moreover, the electrically assisted bicycle configured thus has a relatively heavy motor drive unit that is disposed at a low position at the center of the electrically assisted bicycle (that is, an intermediate point between a front wheel and a rear wheel) in the longitudinal direction. Thus, the front and rear wheels of the electrically assisted bicycle configured thus can be more easily lifted than an electrically assisted bicycle having a motor in the hub of a front or rear wheel. Such an electrically assisted bicycle can easily pass over a step of a path, achieving ease of handling and high traveling stability.
Motor drive units to be provided in such an electrically assisted bicycle are broadly classified into a so-called double-shaft motor drive unit 100 that includes, as shown in FIG. 9, an auxiliary driving force output sprocket 103 that outputs an auxiliary driving force from a motor in addition to a driving sprocket (also called a front sprocket or a chain sprocket) 102 serving as a human driving force output wheel disposed near one end of a crankshaft 101, and a so-called single-shaft motor drive unit 200 shown in FIGS. 10 and 11 in which a human driving force generated by a pedal force and an auxiliary driving force generated by a motor are combined and the resultant force is outputted from a driving sprocket 201.
The double-shaft motor drive unit 100 is disclosed in, for example, Patent Literature 1. As shown in FIG. 9, the auxiliary driving force output sprocket 103 is provided in such a manner as to protrude to the outside of a unit case 104 of the motor drive unit 100 from a portion behind the driving sprocket 102 of the motor drive unit 100. The driving sprocket 102 that outputs a human driving force and the auxiliary driving force output sprocket 103 that outputs an auxiliary driving force are engaged with a chain 105 serving as an endless driving force transmission member. The human driving force and the auxiliary driving force are combined by the chain 105 and then are transmitted to the rear wheel.
Further behind the auxiliary driving force output sprocket 103, a tensioner device (also called a guide device) 106 is provided in engagement with the chain 105, which has been engaged with the auxiliary driving force output sprocket 103, so as to guide the chain 105 downward. Moreover, a tension sprocket 107 provided in the tensioner device 106 increases the winding angle of the chain 105 engaged with the auxiliary driving force output sprocket 103.
Meanwhile, the single-shaft motor drive unit 200 is disclosed in, for example, Patent Literature 2. As shown in FIGS. 10 and 11, the outer periphery of a crankshaft 202 that receives a human driving force transmitted from a pedal has a cylindrical human-power transmission member 203 that receives the human driving force transmitted by serration coupling and so on, and a combined force member 205 where a human driving force transmitted via the human-power transmission member 203 is combined with an auxiliary driving force from a motor 204. Subsequently, the human driving force from the human-power transmission member 203 is transmitted to the combined force member 205 via a one-way clutch 206. A large-diameter gear 205a that receives an auxiliary driving force from the motor 204 via a deceleration mechanism 207 is formed on one end of the combined force member 205, whereas the driving sprocket 201 is attached to the other end of the combined force member 205, the driving sprocket 201 serving as a driving force output wheel engaged with a chain 208 serving as an endless driving force transmission member. A resultant force on the combined force member 205 is transmitted from the driving sprocket 201 to the rear wheel through the chain 208.
As shown in FIGS. 10 and 11, the single-shaft motor drive unit 200 is configured such that the driving sprocket 201 is only engaged with the chain 208 and the resultant force of a human driving force and an auxiliary driving force is transmitted to the chain 208. In contrast, the double-shaft motor drive unit 100 needs to engage, as shown in FIG. 9, the driving sprocket 102 for transmitting a human driving force, the auxiliary driving force output sprocket 103 for transmitting an auxiliary driving force, and the tension sprocket 107 with the chain 105.
Thus, the area of the single-shaft motor drive unit 200 in side view (laterally projected area) can be advantageously smaller (compacter) than that of the double-shaft motor drive unit 100 by devising the layout of the motor 204 and the deceleration mechanism 207. A so-called front derailleur can be easily attached to the single-shaft motor drive unit 200 including the driving sprocket 201 with multiple stages. On the other hand, in the double-shaft motor drive unit 100, the driving sprocket 102, the auxiliary driving force output sprocket 103, and the tension sprocket 107 need to be engaged with the chain 105, leading to difficulty in attaching a front derailleur.
Moreover, the single-shaft motor drive unit 200 advantageously eliminates the need for providing the tensioner device 106 of the tension sprocket 107 or the like. Generally, braking devices used for electrically assisted bicycles include a rim brake, a band brake, and a roller brake that are operated with a brake lever attached to a handle bar as in those of ordinary bicycles. Depending on the regions or the request of an operator, the attachment of a coaster brake to the rear wheel may be required. The coaster brake is operated by rotating the pedal opposite to a forward rotation direction. In this case, however, the pedal rotated in the opposite direction applies a tension that pulls the lower part of the chain forward. Thus, the double-shaft motor drive unit 100 needs a unique design for the tensioner device 106, whereas the single-shaft motor drive unit 200 advantageously eliminates the need for such a unique design.
Typically, in the advantageous single-shaft motor drive unit 200, a magneto-striction torque sensor 209 for detecting a human driving force is provided on the outer periphery of the human-power transmission member 203, which receives a human driving force transmitted from the crankshaft 202, and a portion opposed to the outer periphery. Specifically, a magneto-striction generation portion is formed on the outer periphery of the human-power transmission member 203, and a coil 209a for detecting a change of magnetism on the magneto-striction generation portion is opposed to the magneto-striction generation portion. When the right and left pedals are pressed, the crankshaft 202 is twisted by a pedal force (human driving force). Thus, the twisted state of the human-power transmission member 203 that receives a human driving force transmitted from the crankshaft 202 is detected by the torque sensor 209.
The magneto-striction generation portion of the torque sensor 209 is formed on the outer periphery of the human-power transmission member 203 while the one-way clutch 206 is attached to the end of the human-power transmission member 203 as discussed above. The one-way clutch 206 is provided for the following reason: even if a rider stops pedaling on such a conventional electrically assisted bicycle, the motor 204 is controlled so as to keep rotating for a while (so-called delay control). In this case, the absence of the one-way clutch 206 may transmit an auxiliary driving force from the motor 204 to the crankshaft 202, causing the pedals to rotate without being pedaled by the rider. Thus, the one-way clutch 206 interrupts an auxiliary driving force from the motor 204 so as not to apply such a force to the crankshaft 202 or the pedals.
As shown in FIG. 11, a one-way clutch 210 is also provided between a rotating shaft 204a and a rotor 204b of the motor 204 in the single-shaft motor drive unit 200. If a battery for driving the motor 204 has run out during pedaling, the one-way clutch 210 eliminates the need for rotating the rotor 204b of the motor 204. Specifically, if the battery has run out during pedaling, the absence of the one-way clutch 210 rotates the rotor 204b of the motor 204 with a pedal force of the pedal. Thus, the cogging torque or the like of the motor 204 requires a large force for rotating the pedals (so-called drag resistance). To address this problem, the one-way clutch 210 is provided to eliminate the need for rotating the rotor 204b of the motor 204. This eliminates the need for applying an excessive force generated by, for example, the cogging torque of the motor 204.
Patent Literature 3 discloses an example of a single-shaft motor drive unit. As shown in FIG. 12, a single-shaft motor drive unit 250 includes a cylindrical combined force member 255 disposed on the outer periphery of a crankshaft 251 that receives a human driving force transmitted from the pedal. The combined force member 255 receives a human driving force transmitted from the crankshaft 251 through a one-way clutch 252 attached to one end of the combined force member 255 and receives an auxiliary driving force transmitted from a motor 253 through a deceleration mechanism 254 or the like. Moreover, a large-diameter gear 259 engaged with an output gear 254a of the deceleration mechanism 254 is attached to another one-way clutch 258 near the other end of the combined force member 255. An auxiliary driving force from the motor 253 is transmitted to the combined force member 255 through the deceleration mechanism 254, the large-diameter gear 259, and the one-way clutch 258, and then a resultant force combined on the combined force member 255 is transmitted from a driving sprocket 257 to the rear through a chain 256.
In the single-shaft motor drive unit 250, the magneto-striction generation portion is formed on the outer periphery of the combined force member 255 that receives a human driving force transmitted from the crankshaft 251 and an auxiliary driving force transmitted from the motor 253, and magneto-striction torque sensors 260 are provided. The torque sensor 260 has a coil that is opposed to the magneto-striction generation portion to detect a change of magnetism on the magneto-striction generation portion. When the right and left pedals are pressed, the crankshaft 251 is twisted and thus the twisted state of the combined force member 255 that receives a human driving force transmitted from the crankshaft 251 is detected by the torque sensor 260.
In the single-shaft motor drive unit 250 of FIG. 12, the one-way clutch 252 is attached to the one end of the combined force member 255. Thus, even if a rider stops pedaling, the motor 253 keeps rotating for a while. Also in this case, an auxiliary driving force from the motor 253 is interrupted by the one-way clutch 252 so as not to be applied to the crankshaft 251 or the pedals.